In digital communication systems, information is transferred between sending and receiving terminals over a transmission line or, more generally, a communication channel in a format suitable for that channel. The format can be a baseband signal or, using a digitally modulated carrier, a bandpass signal. In either case, the signal is characterized by a fundamental baud or symbol rate, i.e., the clock frequency of the data.
In synchronous digital communications, it is frequently necessary to extract a coherent clock signal from a received input data stream. This process is known as clock recovery. The recovered clock enables the receiving terminal to optimally sample the received digitally encoded communication signals. Clock recovery circuits commonly are employed, for instance, in conjunction with analog data communications over telephone or radio channels.
Conventional clock recovery circuits typically employ phase-locked loops ("PLL's"). A typical PLL has a voltage-controlled crystal oscillator (VCXO) whose phase is locked onto a frequency component of its input signal. The PLL also has a phase detector for comparing the phase of the oscillator with the phase of the input signal, and for developing a voltage proportional to the phase differential, which commonly is referred to as a "phase error". This voltage is filtered and applied as a control voltage to the oscillator so as to adjust the oscillator's frequency. Due to negative feedback, the phase error is driven to a value which is preferably small, and the oscillator's frequency consequently is kept equal to the input frequency. However, the aforementioned VCXO is disadvantageous in that it is not versatile (it is not readily adaptable in other circuit schemes) and has a long delivery time on frequency. Further, high resolution Digital-to-Analog converters which are required to drive VCXO's are complex and expensive to implement.
In contrast, crystal oscillators provide both inherent stability characteristics and do not need the requirement of the aforementioned Digital-to-Analog converter required to drive the voltage controlled crystal oscillator.
It is thus an object of the present invention to provide a variable phase shifter which employs a crystal oscillator to be incorporated into clock extraction circuitry and is adaptable for the implementation of a wide range of crystal oscillators.